Historically, high quality, high power, power amplifiers, used for home high-fidelity reproduction have been large, heavy and expensive. For example, a typical 400 watt amplifier weighs anywhere from 35 lbs. to over 85 lbs. depending upon the particular design and choice of materials. Such prior art power amplifiers are heavy for two reasons. First, they require a power transformer that makes up about 60% of the weight of the total amplifier. Second, they require heat sinks which make up about 25% of the total weight. Thus, about 85% of the amplifier's weight is contributed by the power transformer and the heat sinks.
Another consideration is that the power which is dissipated in the heat sinks represents wasted power. In a typical prior art power amplifier the efficiency generally averages around 50%, with the efficiency being higher when the amplifier is reproducing signals of a larger magnitude, and the efficiency being quite small when signals of small magnitude are being amplified.
This latter problem has been recognized in the prior art, and a number of patents relating to audio amplifiers disclose devices which have for their object the purpose of limiting the voltage supply to the power input terminals to the amplifier itself to avoid wasting current. For example, in U.S. Pat. No. 3,426,290, Jensen, there is a control circuit in series between the output terminals of a power transformer and the load, which is the amplifier itself. A control signal is derived from the audio input signal, and this controls a transistor in series between the output of the transformer and the amplifier to turn the transistor on and off to chop up the current which goes to the load. This arrangement requires a series connected inductance coil and a shunt capacitor between the load and the control transistor. A later patent, U.S. Pat. No. 4,054,843, Hamada, operates in essentially the same way.
Another approach to this same general problem of improving efficiency in audio amplifiers is shown in U.S. Pat. No. 3,319,175, Dryden, which discloses a device which supplies a varying voltage to the amplifier. However, the voltage changes in increments, and this is accomplished by placing multiple tabs on the output side of the power transformer. A control signal is derived from the audio signal to tap various parts of the transformer as needed to increase or decrease the voltage applied to the amplifier. Similar devices are disclosed in U.S. Pat. No. 3,772,606, Waehner, and in U.S. Pat. No. 3,961,280, Sampei.
U.S. Pat. No. 3,585,517, Herbert, also relates to a power amplifier. This patent discloses a device which converts an audio signal to a series of pulses, the amplitude of which is proportional to the input signal. These pulses are then reconstructed back into an audio signal in the amplifier.
A number of other patents were disclosed in a search of the patent literature. These are not considered relevant to the present invention, but are disclosed herein mainly to insure that the applicant is complying with his duty of making a full disclosure of anything which might be considered relevant as prior art. These following patents relate to controlling the voltage of a power source, with the voltage being related to some absolute steady reference voltage to provide a constant voltage to the load. These patents are as follows: U.S. Pat. Nos. 3,396,326, Kisrow; 3,564,393, Williamson; 3,581,187, Grady Jr.; 3,740,640, Ravas; 3,745,443, Weil; 3,863,140, Easter et al; 3,873,903, Koetsch et al; 3,931,567, Kostecki.